It is known a type of bearings, so called plastic bearing, which is made of plastic such as polyamide resin, phenol resin, or polyphenylene sulfide mixed with a modifier such as graphite, carbon fiber, glass fiber, metal powder or carbon black (refer to, for example, S. Yamaguchi; Lubricity of Plastic Materials, Nikkan Kogyo Shimbun). Such plastic bearings are widely used for their easy maintenance as they require no lubrication.
These plastic bearings vary in their characteristics depending on the type and amount of modifier blended to plastic matrix. Different modifiers are used depending on their purposes such as wear resistance, slidability (frictional characteristics), and heat resistance.
Addition of carbon black, for example, for the purpose of improved wear resistance, however, often deteriorates the slidability significantly. Addition of expansive graphite that is excellent in lubiricity and lubiricant absorbency, for the purpose of improved slidability, on the other hand, results in inferior wear resistance. Addition of metal powder for the purpose of improved heat resistance may result in degraded slidability. In the conventional plastic bearings, an attempt to improve a given property inevitably results in degradation of another property due to defects of a modifier added and it was difficult to attain a high level in all of the required characteristics such as wear resistance, slidability and heat resistance. In fact, there was no modifier which could meet the requirements by itself.
Moreover, the conventional plastic bearings are usually supported by a metal holder or a housing. When a plastic bearing is press-fitted or inserted to a metal holder, plastic becomes contracted or deformed due to the pressing force, and eventually the bore dimension becomes altered by a great margin, causing a significant deterioration in the bearing function. Plastic blended with an inorganic modifier such as graphite, carbon fiber, glass fiber, metal powder, or carbon black at a high content tends to generate contraction of the bore size as well as cracks in the form of bearings due to pressing. The cracks would result in tearing of the sliding face to thereby accelerate wear. Even if improved wear resistance, slidability (low friction coefficient) and heat resistance could be achieved in terms of the composition by addition of modifiers, such characteristics might be offset by dimensional errors or cracks occurring during the manufacturing and processing steps. In the end, the bearing function itself would become deteriorated. There is known a sliding member made of vitreous carbon material which contains a solid lubricant graphite (JPA Laid Open No. Sho 59-131567). However, a mixture of vitreous carbon material and graphite is sintered and carbonized to form a sliding member material, and a significant contraction occurs at the time of sintering and carbonizing of the mixture. Moreover, as the fluctuation in the degree of contraction is significant, it is impossible to obtain bearings with satisfactory size precision.